Installation, 2013

Helen Mac Mahon (IE)

Motion Aftereffect Illusion consists of two discs with a spiral image capable of inward or outward motion. In between these two discs is a mirror. When the viewer looks at the centre of one of the rotating spirals for several seconds, then looks at any stationary pattern, it appears to be moving in the opposite direction. This form of the motion aftereffect is known as the spiral aftereffect. In this piece, the viewer looks at one of the spirals and then looks at their face in the mirror. Depending on the direction that the spiral is going, the motion aftereffect will create the illusion that their head is either shrinking or growing bigger.

Like all aftereffects, this one can be explained in terms of the ‘fatigue’ of nerve cells encoding for one motion direction. When motion detectors in the brain detect movement they produce a strong signal, and when there is no movement detected they produce a weaker (but non-zero) signal. When these detectors are strongly stimulated, they fatigue after several seconds of continuous firing activity. This can also be described as the motion detectors ‘adapting’ to the continuous movement. When the stimulus is removed, or the movement stops, the motion detectors in the opposite direction produce a stronger signal for a few seconds as a result of the balance between these groups being disrupted. The effect disappears when the fatigued motion detectors recover. MOTION AFTEREFFECT ILLUSION Installation, 2013 Helen Mac Mahon (IE) 24 ILLUSION—

Artist's Statement

The motion aftereffect is the illusion of motion in the visual image caused by prior exposure to motion in the opposite direction. For example, when one looks at rocks beside a waterfall they may appear to drift upwards after the viewer has looked at the flowing water for a period of about 60 seconds. The illusion originates in the visual cortex, and arises from selective adaptation in cells tuned to respond to movement direction.